FIG. 10 illustrates a typical conventional radio frequency transceiver. The radio frequency transceiver includes an antenna 1, a radio frequency pre-select filter (band-pass filter (BPF)) 2, a duplex switch 3, a receiving channel, and a transmitting channel. The antenna 1 is connected to the channel for a receiving device 1004, or to the channel for a transmitting device 1005, via the duplex switch 3. The radio frequency transceiver is used to convert a base-band (BB) data (signal) into a radio frequency signal. Generally, in terms of a signal energy frequency, a base-band signal has a frequency range of direct current to several tens of MHz, whereas a radio frequency carrier wave has a frequency region of GHz. More specifically, for example, a wireless local area network (WLAN) transceiver is compliant with the IEEE 802.11b standard. In such a WLAN transceiver, a base-band signal has a frequency range of direct current to 11 MHz, whereas a radio frequency carrier wave varies depending on a selected channel, i.e., has a frequency range of 2412 MHz to 2484 MHz.
Meanwhile, there are several methods for carrying out a frequency conversion from a base-band signal to a radio frequency signal in the transmitting channel, and a frequency conversion from a radio frequency signal to a base-band signal in the receiving channel. All of the methods employ one or more mixers for upconversion or downconversion of frequency.
FIG. 11 illustrates a well-known heterodyne (or a super-heterodyne) system in which a two-stage downconversion into a base-band signal is carried out with respect to a radio frequency signal (i.e., a received signal superimposed on a radio frequency carrier wave) in a receiving channel. Specifically, in the first stage downconversion, the received signal, on the radio frequency carrier wave having a frequency fRF, is amplified by a low noise amplifier (LNA) 4. The received signal thus amplified are mixed, by a first mixer 5, with an output signal having a frequency fLO that is sent from a first local oscillator (LO). The oscillator (LO) is made up of a phase-locked loop (PLL) circuit PLL1 and a voltage controlled oscillator (VCO) VCO1. The mixing allows a downconverted signal, which has been subject to the first stage downconversion and has a frequency of f1 (f1=fRF−fLO), to be outputted via an output terminal of the first mixer 5. Generally, the radio frequency transceiver includes a band-pass filter 6 or an LC load so that an interference signal or noise is removed from a frequency band near the frequency f1. The LC load is disclosed, for example, in K. L Fong, C. D. Hull and R. G. Meyer, “A Class AB Monolithic Mixer for 900-MHz Applications”, IEEE Journal of Solid-State Circuits, vol.32, pp.1166-1172, August 1997 (hereinafter, referred to as “non-patent document 1”). Note that, in the WLAN IEEE 802.11b transceiver, the frequency f1 is approximately 374 MHz.
In the second stage downconversion, a carrier wave having the frequency f1 is mixed, by a second mixer 7, with an output signal, having a frequency f1F, that is sent from a second local oscillator made up of a phase-locked loop circuit PLL2 and a voltage controlled oscillator VCO2. This mixing allows generation of a base-band signal. Note that, in this example, the output signal of the second local oscillator has the frequency fIF of 374 MHz. The second mixer 7 outputs a base-band signal having a frequency fBB. Thereafter, the base-band signal is filtered by a low-pass filter 8, and then the signal thus filtered is amplified by an amplifier 9. The signal thus amplified is converted into a digital signal by an analog/digital converter (ADC) 10. After that, a digital processing is carried out with respect to the digital signal.
On the other hand, in a transmitting channel, a digital base-band signal is converted into an analog base-band signal by a digital/analog converter (DAC) 11. Then, the analog base-band signal is filtered by a low-pass filter 12 so that interference is reduced, and so that bandwidth of the analog base-band signal is controlled. The signal thus filtered is mixed, by an intermediate frequency (IF) mixer 13, with an output signal sent from the second local oscillator. This allows upconversion of the signal from base band to the frequency f1. The signal thus upconverted is filtered by a band-pass filter 18. The signal thus filtered is mixed, by a mixer 14, with an output signal of the first local oscillator so as to be upconverted again. The signal thus upconverted is amplified by a power amplifier (PA) 17 so as to obtain sufficient electric power for driving the antenna 1. Note that a band-pass filter 16 is provided between the mixer 14 and the power amplifier 17.
FIG. 12 illustrates how a frequency spectrum changes in accordance with the downconversion of the heterodyne transceiver.
Here, there is a signal having a frequency which keeps a frequency of a desired radio frequency signal away from the frequency of the output signal of the first local oscillator. During the mixing processing of the first mixer 5, such a signal is downconverted into a signal having the frequency f1, like the desired radio frequency signal.
This is the well-known weakness of the heterodyne system. As shown in FIG. 12, when the desired frequency fRF of the radio frequency signal is higher than the frequency fLO of the output signal of the first local oscillator (i.e., fRF>fLO), there is an unwanted signal having a frequency fIM (fIM<fLO). The unwanted signal having the frequency fIM is called an “image signal” in technical term, and satisfies the following relation:fRF−fLO=fLO−fIM The image signal is downconverted into a signal having the frequency f1 like the desired radio frequency signal. (The frequency fIM of the image signal is called an “image frequency”.) Specifically, in the WLAN IEEE 802.11b transceiver, if it is assumed that the frequency fLO of the first local oscillator is 2038 MHz and the frequency fRF of the desired radio frequency signal is 2412MHz, then the image frequency fIM is 1664 MHz.
In order to avoid interference of the image signal, used is a direct conversion transceiver shown in FIG. 13 (also known as a homodyne transceiver). The direct conversion transceiver carries out, in a single mixing step, a frequency conversion from a radio frequency signal to a base-band signal, and a frequency conversion from a base-band signal to a radio frequency signal. For example, in a receiving channel of the transceiver, a radio frequency signal having the frequency fRF is amplified, and the signal thus amplified is mixed, by a mixer 21A and a mixer 21B, with local oscillation signals each having the frequency fLO (fLO=fRF). This allows the radio frequency signal to be downconverted into a signal having a base band component.
As such, the direct conversion transceiver can prevent the problem of the image signal.
Meanwhile, as a substitution of the heterodyne transceiver, disclosed in U.S. Pat. No. 6,351,502 B1 (published on Feb. 26, 2002; hereinafter, referred to as “patent document 1”) is so-called a dual conversion transceiver. FIG. 14 illustrates the dual conversion transceiver.
The dual conversion transceiver is one of the heterodyne transceivers because two-stage conversion of a radio frequency signal to a base-band signal is carried out by a first mixer 106 and I/Q (I: in-phase, Q: quadrature) mixers 108A and 108B.
The dual conversion transceiver is also easily affected by the image signal having the frequency fIM (fIM=2fLO−fRF). In the dual conversion system of the patent document 1, the frequency fRF is 5.2 GHz, the frequency fLO is 4.24 GHz, and the image frequency fIM is 3.28 GHz. This allows the image frequency fIM to be out of band of a low noise amplifier 105. Accordingly, the image signal is greatly attenuated.
Meanwhile, another dual conversion transceiver is disclosed in A. Zolfaghani and B. Razavi, “A Low-Power 2.4-GHz Transmitter/Receiver CMOS IC, IEEE Journal of Solid-State Circuits, vol.38, pp. 176-183, February 2003 (hereinafter, referred to as “non-patent document 2”). FIG. 15 illustrates a structure of such another dual conversion transceiver.
Further, a circuit (see FIG. 16) is disclosed as a double conversion tuner in Japanese Laid-Open Patent Application Tokukai 2000-299646 (published on Oct. 24, 2000; hereinafter, referred to as “patent document 2”).
A first mixer 402 mixes a radio frequency signal with a clock signal (local oscillation signal) having a frequency fLO1 that is sent from a voltage controlled oscillator 406. This allows the downconversion of the radio frequency signal. A second mixer 404 mixes (i) the signal thus obtained by downconverting the radio frequency signal by using the frequency fLO1 with (ii) a clock signal having a frequency fLO2 that is received from a crystal-controlled oscillator (XCO) 412. The two clock signals satisfy the following relation:fLO1=fLO2×(N/M).
However, the conventional radio frequency transceivers have a problem in being fully mounted in an integrated circuit (IC) or an IC chip as follows.
Specifically, the heterodyne radio frequency transceiver requires a bulky filter, provided outside the IC chip, for restraining the image signal. This results in that the heterodyne radio frequency transceiver cannot be fully mounted in the IC chip (integrated circuit). Further, in order to drive the surface acoustic wave filter provided outside the IC chip, it is necessary to output, to the outside of the IC chip, the radio frequency signal amplified by a low noise amplifier. As such, it is necessary for the heterodyne radio frequency transceiver to further include a buffer amplifier. This causes an increase in power consumption of the IC chip (integrated circuit).
As for the heterodyne radio frequency transceiver shown in FIG. 11, the heterodyne radio frequency transceiver additionally includes a band-pass filter 15 so that the image signal is attenuated. The band-pass filter 15 is made up of bulky and discrete components so that an image signal removing characteristic is obtained. The band-pass filter 15, however, hinders the heterodyne radio frequency transceiver from being mounted in an integrated circuit. This causes the heterodyne radio frequency transceiver to be expensive, and to have extra power consumption.
The patent document 2 relates to a double conversion system that is applied to a tuner. Because the double conversion system is basically one of the heterodyne systems, the double conversion system also has the foregoing problems (i.e., the impossibility of the full mounting in an integrated circuit, and the increase in the power consumption of the integrated circuit).
According to the system disclosed in the patent document 2, the frequency fLO2 of the second local oscillation signal used by the second mixer 404 is fixed. This means that the frequency fLO2 does not change according to the frequency fLO1 of the first local oscillation signal used by the first mixer 402. Therefore, when the frequency fLO1 of the first local oscillation signal changes, a ratio of the frequency fLO1 of the first oscillation signal to the frequency of the second oscillation signal deviates from an initial setting value.
Further, according to the system of the patent document 2 the following relation is satisfied:fLO1−fLO2=1.0101×fRF This means that the downconversion in the patent document 2 is not a downconversion to direct current, but a downconversion to a second intermediate frequency (IF) band (2.048 MHz). Therefore, the double conversion tuner of the patent document 2 is not applicable to a transceiver for carrying out a downconversion from a radio frequency signal to a base-band signal.
Furthermore, when the intermediate frequency fLO2 is 800 MHz, the double conversion system of the patent document 2 requires a voltage controlled oscillator controlled by the phase-locked loop circuit, instead of the crystal-controlled oscillator.
Although the direct conversion system solves the problem of the image frequency, the direct conversion system has deterioration in sensitivity. Further, the direct conversion system has a dynamic direct current offset at an output terminal of the mixer. This is because the signal of the local oscillator leaks into a radio frequency port, and is subject to self-mix. Generally, in the direct conversion system, it is necessary for the low noise amplifier to have a high gain. This causes a possible increase in power consumption.
Meanwhile, also easily affected by the image signal are the conventional dual conversion transceivers (see FIG. 14 and FIG. 15) disclosed in the patent document 1 and the non-patent document 2, respectively. As such, normally, it is necessary to include a kind of bulky filter for restraining the interference of the image signal. Further, the lower the frequency of the image signal becomes, the larger the filter becomes. As such, as the frequency of the radio frequency signal becomes lower (≦2.4 GHz), this kind of problem goes worse (i.e., the filter occupies a much larger area).
According to the transceiver disclosed in the non-patent document 2, it is necessary to remove the image signal with the use of the filter on the IC chip. In the transceiver disclosed in the non-patent document 2, the following relations are satisfied by the frequency fRF of the radio frequency signal, the frequency fLO of the local oscillation signal used for the first stage downconversion, the frequency fIF of the intermediate signal obtained by the first stage downconversion, and the frequency fIM of the image signal:fRF=2.4 GHz;fLO=(2/3)fRF;fIM=fLO−fIF Therefore, the frequency fIM of the image signal is 800 MHz. Because the image signal has such a low frequency fIM of 800 MHz, the filter on the IC chip normally needs a larger area.
Further, the patent document 1 suggests that the frequency fLO1 of the first local oscillation signal LO107 (i.e., the signal LO107 supplied to the first mixer 106 in the patent document 1) is changed into a higher frequency than the frequency fRF of the radio frequency signal. However, because the transceiver disclosed in the patent document 1 uses local oscillation signals I and Q, having phases of 0° and 90°, which are supplied to the I/Q mixers 108A and 108B, respectively, the transceiver does not appropriately operate. Namely, if signals having phases of 0° and 90° are used as the second local oscillation signals LO2 (I and Q), respectively, under the condition of fLO1>fRF, then the radio frequency transceiver does not appropriately operate.
Here, this problem is explained in detail. Firstly explained is a change in frequency spectrums when fLO1<fRF is satisfied. The radio frequency signal has a certain frequency component which is included in a side band (i.e., a frequency component higher than the carrier wave). Such a frequency component still stays higher than the carrier wave even after the first stage downconversion. Under the circumstances, if the second stage downconversion is carried out, by using the local oscillation signals having phases of 0° and 90°, with respect to the frequency component, then an extracted base-band signal has a counterclockwise voltage vector (i.e., a rotation direction which is headed from I (0°) to Q (90°)). Therefore, modulation is properly accomplished.
Next, when fLO1>fRF is kept satisfied, the first stage downconversion causes the frequency component which is included in the side band to be lower than the carrier wave. Under the circumstances, if the second stage downconversion is carried out, by using the local oscillation signals having phases of 0° and 90°, with respect to the frequency component, then an extracted base-band signal has a clockwise voltage vector (i.e., a rotation direction which is headed from Q (90°) to I (0°)). Therefore, modulation cannot be properly accomplished.